Optical fibers offer greatly increased bandwidth, transmission capability and transmission characteristics over traditional copper wires. Use of optical cables has generally been limited to large scale long haul trunking installations, such as those of the telecommunications industry, where the improved transmission characteristics of the optical fibers justify the greater expense and typical difficulty associated with their manufacturing and installation. Nevertheless, as demands on communication media and data volume continue to increase, the advantages of using optical cable for transmission of signals across shorter distances, or for interconnecting local devices, continues to grow. With this growth has come a need to connect fiber optic cables accurately and economically to each other and to a multiplicity of devices. The ability to accurately connectorize optical fibers used for data transmission with electronic devices has become vital to the design of many opto-electronic applications.
The use of optical fibers and the coupling of the optical fibers with electronic devices do present some difficulties. Optical fibers are hair-thin waveguides that conduct light signals. To avoid losing or degrading the light signals being transmitted, there is a need for precise alignment and coupling any time optical fibers are connected to each other or to opto-electronic or optical devices. Optic transfer efficiency is the term used to measure the ability of a connector to accurately couple the transmitted signals.
Of considerable relevance to the problem of developing practical fiber optic connectors is the question of the optic transfer efficiency at the connector. Various factors affect the optic transfer efficiency at a connector including (a) gap separation at the point of abutment, (b) lateral separation due to axial misalignment, and (c) thermal expansion characteristics of connectors.
Numerous optical cable connectors have been developed to aid in the connection of fiber optic cables. As data requirements grow, single fiber cables have given way to high-precision multiple fiber cables, such as parallel ribbon cables including a plurality of optical fibers aligned in parallel. As the number of fibers grow, such do the difficulties in maintaining the transfer efficiency of the connector.
Examples of known multi-fiber connectors include the MAC.TM. connector by Berg Electronics and the MT Connector by U.S. Conec. Further examples of optical connectors are illustrated in U.S. Pat. No. 5,420,952 to Katsura, et al.; U.S. Pat. No. 5,276,755 to Longhurst; U.S. Pat. No. 5,500,915 to Foley et al.; U.S. Pat. No. 4,784,457 to Finzell; U.S. Pat. No. 5,430,819 to Sizer, II, et al.; and U.S. Pat. No. 5,287,426 to Shahid.
Many of the known connectors have disadvantages associated with them. An MT-type connector, a portion of which is illustrated in FIGS. 1A and 1B, is one of the most common connectors currently used. MT Connector 10 includes a first alignment block, a ferrule 12, and a second alignment block, and a receptacle or mating ferrule 14. The term alignment block is meant to include ferrules, receptacles, or any other mating blocks. The ferrule 12 has protruding long pins 20. The proposed TIA/EIA-604-5 MT connector intermateability standard specifies that the alignment pins must protrude at least 2.285 pin diameters (1.6 mm protrusion for a 0.7 mm diameter pin) from the face of the ferrule.
Long thin pins, such as those of the MT connector, attempt to control movement of the connector in the x, y and z axes. Long pins may help achieve suitable optical connections for some applications and the coupling of pins and holes may be intuitive to users. However, the use of such long pins does present significant coupling, alignment, durability and manufacturing disadvantages.
As illustrated in FIG. 1, during coupling of an MT-type connector, the ferrule 12 is interference fit upon a receptacle 14. The receptacle 14 defines a receiving orifice or hole 30. The pin 20 is inserted into the corresponding receiving hole 30. Significant insertion force is needed to seat each small diameter (.about.0.7 mm) pin fully into the respective hole. It has been calculated that the interference fit of a nominal MT connector pin inserted into a matching receptacle hole could require approximately six Newtons of force to fully seat. If the pins are not fully seated, an air gap between the two mating alignment blocks results that can cause severe light loss.
Correct alignment of the pins is very important before coupling. FIG. 1A illustrates a 0.5 mm lateral misalignment of the 0.7 mm MT connector pin 20. The small diameter of the pin 20 and of the matching receiving hole 30 results in complete failure to couple even under very small (e.g., half a millimeter) lateral misalignment.
FIG. 1B illustrates the effects of angular misalignment of pin 20. The effects of even a small angular misalignment are magnified by the length of the pin, even a small angular misalignment (e.g., 5 degrees) may again result in complete failure to couple.
If the pin 20 is not perfectly aligned before engagement into the mating hole 30, the pin 20 may miss the hole 30 and crack the mating ferrule 14 causing a catastrophic failure. The long and thin metal pins 20 and 22 also are liable to bend during insertion and withdrawal and damage the mating ferrule 14 on subsequent insertions. The high interference fit of the long pin to the mating hole can cause the hole to be "skived" and deposit unwanted debris onto the connector mating face, which can cause signal failure. Because the pins protrude so far from the mating face of the MT, the mating face is difficult to clean.
Manufacture of an MT connector further requires tight control of the tolerances of at least nine critical dimensions: (1) pin diameter, (2) pin straightness, (3) pin taper, (4) hole diameter, (5) hole straightness, (6) hole angle, (7) hole taper, (8) hole placement relative to matching hole, (9) hole placement relative to fibers. Accordingly, the use of traditional alignment pins further drives up manufacturing difficulty and costs.
A further consideration is that the long protruding metal MT alignment pins have a tendency to act as "antennas" and may cause electromagnetic interference when placed near high frequency components. This interference may in turn cause signal interference to other equipment and components.
An alternative optical connector design is disclosed in U.S. Pat. No. 5,778,123, entitled "Alignment Assembly for Multifiber or Single Fiber Optical Cable Connector", which is commonly assigned with the present invention to Minnesota Mining and Manufacturing and which is hereby incorporated by reference. The patent discloses a "ball and socket" alignment structure where an opening or socket in a ferrule seats a ball, rather than a long pin. The opening has a depth d.sub.1. The ball has a radius R, where R&gt;d.sub.1. The ball and socket structure offers significant advantages as the design does not overconstrain the z-axis alignment and requires control of only two manufacturing tolerances: the size of the alignment ball, which is easily controllable, and the spacing between the two openings.
However, the ball offers only a limited bonding surface to the associated alignment hole. A limited bonding surface may result in inadequate bonding of the ball to the ferrule. Also, the ball and socket design may be susceptible to damage from overpolishing of the ferrule and fiber ends. Overpolishing a ball-in-socket ferrule face may damage or obliterate the ball alignment opening or chamfer, thus inhibiting accurate attachment of the ball.
Another issue with traditional connectors has been the electrical interconnection of opto-electronic components embedded in a receptacle. Manufacturing limitations encourage the placement of the grounding contact for traditional VCSEL on a back plane of the integrated circuit (see, e.g., "High-Performance, Producible Vertical-Cavity Lasers for Optical Interconects", R. Morgan, Intl. J. of High Speed Elect. and Syst., Vol. 5, No 4 (1994), relevant portions of which are hereby incorporated by reference). Once the opto-electronic component is embedded in a receptacle, a robust electrical connection to the grounding back-plane becomes difficult.
The opportunity remains for an improved optical and opto-electronic connector assembly.